The present invention relates to a pretensioner for a vehicle seat belt system and, in particular, to a pretensioner of the type in which the belt is tightened by rotating a belt reel of a retractor in the belt-winding direction.
In some vehicle seat belt systems, a belt pretensioner is provided for the purpose of preventing a secondary collision of an occupant of the vehicle by operating in an emergency and by tightly restraining the body of the occupant on the seat of the vehicle.
Japanese Provisional Patent Publication No. 119454/1989, for example, describes and shows a pretensioner in which a belt is wound and tightened by rotating a pulley that is connected, upon operation of the pretensioner, to a belt take-up shaft of a seat belt retractor by a coupling unit. The pulley is rotated by a cable coupled to a driving power source and wound onto the pulley. The coupling unit comprises a pair of gears, one of which is fixed on the take-up shaft and the other of which is fixed on the pulley. The pulley is supported on a movable support member that is ordinarily held in a position such that the gears do not mesh by a shear pin, so the retractor can operate normally, without interacting with the pretensioner, except in an emergency that results in operation of the pretensioner. When the operation of the pretensioner is triggered, the gears are engaged with each other upon shearing of the shear pin and movement of the pulley support member.
In the above-described known device, the rotation of the gear on the pulley must be restrained until the gears are engaged in proper mesh. Accordingly, a shear pin having an accurate shear load operating in two steps is required. Also, because a very large load is exerted on the pulley instantaneously (on the order of milliseconds), a slight change in resistance to shear may have a large influence on the operation of the system. Also, it is necessary to have a strong support member to carry the operating load, and a power loss is involved in freeing the guiding unit from restraint by the shear pin.